Integrated circuits are made in a bulk parallel process by patterning and processing semiconductor wafers. Each wafer contains many identical copies of the same integrated circuit referred to as a “die.” Semiconductor wafers must be tested before the die is cut into individual integrated circuits and packaged for sale. If defects are detected, the defective die can be culled before wasting resources packaging a defective part.
To test a wafer, a probe card is commonly used to come in contact with the surface of the wafer. The probe card generally contains three unique characteristics: (1) an XY array of individual probes that bend in the Z direction to allow contact with the die; (2) an electrical interface to connect the card to a circuit test apparatus; and (3) a rigid reference plane defined in such a way that the probe card can be accurately mounted in the same location on the wafer tester. FIG. 14A illustrates a probe card (1405) that contains twenty single die probe areas (1410) allowing it to test twenty die simultaneously. FIG. 14B is a callout figure of the probe card (1405) and shows a single die probe area (1415) that contains two arrays of several individual probes (1420). The particular configuration of the probes depicted in FIG. 14B would correspond to the placement of the contact pads on the integrated circuit die under test (also known as the device under test or “DUT”).
FIG. 15 depicts a cross sectional view of a possible structure for an individual probe. The probe may comprise a substrate (1505), probe base (1510), spring element (1515), tip supporting structure (1520), and a probe tip (1525). When the probe card is brought in contact with the die, the Z-direction bending (shown as arrow 1530) allows for an electrical contact between the probe tip (1525) and the contact pad of the DUT. Other probe structures are possible including the “Torsion Spring Probe Contactor Design” disclosed in U.S. patent application Ser. No. 11/194,801 commonly owned by the present applicant, the contents of which are incorporated herein by reference. The torsion spring contactors described therein may be manufactured using the lithography techniques described in U.S. patent application Ser. Nos. 11/019,912 and 11/102,982, both commonly owned by the present applicant and hereby also incorporated by reference.
The probe card ultimately provides an electrical interface that allows a circuit test apparatus to be temporarily connected to the individual DUT. This method of testing is extremely efficient because many die can be tested at the same time. To drive this efficiency even higher, probe card manufacturers are making larger probe cards with an ever-increasing numbers of DUTs and thus probes.
As the probe comes into contact with the contact pads of the DUT, the probe exerts a force on the DUT in the Z-direction (i.e., perpendicular to the plane of the DUT) and a smaller but still significant force in the lateral direction (i.e., parallel to the plane of the DUT). FIG. 16 illustrates the forces that a probe (1605) may exert on a contact pad (1610) of the DUT. The probe (1605) exerts a lateral force Flat in the direction of vector 1615, which has a component Fx in the X direction as shown by vector 1620 and a component Fy in the Y direction as shown by arrow 1625. The magnitude of Flat=(Fx2+Fy2)1/2 and the angular direction is tan−1 Fy/Fx.
Many semiconductor devices (particularly memory devices) are tested in parallel (i.e. many die at a time as shown in FIG. 14A) and have unequal numbers of contact pads on top vs. bottom and/or right vs. left sides of the die. The probe card used to test such a DUT would necessarily have an uneven distribution of probes that match the contact pads. This is the case shown in FIG. 14B, where the top array of probes has fifteen individual probes, while the bottom array contains twenty-five probes. As a consequence, the individual probes within a single die probe area exert a force that is not balanced, resulting in each individual die probe area experiencing a net lateral force. In FIG. 17, a probe card (1705) is shown with the same single die probe area (1710) configuration as in FIG. 14B. Because of the imbalance within one of these areas, a vector 1715 develops within each area. These vectors form a net lateral force vector 1720 for the entire probe card.
As the semiconductor industry has continued to decrease the size of integrated circuits (and consequently the die size), both the contact pad size and their spacing have decreased accordingly. Thus, new manufacturing methods have utilized photolithographic and micro-machining techniques to very accurately position the probes within the needed tolerances and to pack more probes onto a single card. Current state of the art semiconductor manufacturing routinely produces contact pad sizes of 80 um with inter-pad spacing on a 100 um pitch. A current probe card may have as many as 5,000 or more individual probes that engage the contact pads of the DUTs. Consequently, the net lateral force over these probe cards has also increased significantly. The increase in lateral force places the probe card and the DUT under stress; this can compromise the efficacy of the probe testing and may damage the DUT.
Currently available techniques to address the net lateral force include orienting the probes in roughly the four orthogonal directions, such that the lateral forces could be diminished. The probe card may also include support screws and stiffener elements to minimize the mechanical effect of net lateral force on the probe card. In addition, the probe card may be moved vertically and laterally to ameliorate the lateral forces exerted by the probe onto the DUT. Finally, the support structure for the wafer (also known as the chuck) may be engineered to be extremely rigid laterally.
Each of these techniques has shortcomings. For example, the rough orientation of the probes often cannot work in probe card layouts because contact pads for the DUT do not lend themselves to a completely orthogonal layout. Adding rigidity, either to the probe card or the chuck, adds expense to the probe testing process. More importantly, these techniques do not address the fundamental problem; rather they are band aid solutions that become less and less effective as the number of probes on a card continues to explode.
What is needed, therefore, is a method and apparatus that minimizes or neutralizes the root cause of the net lateral force.